The reaction with styrene with twice the molar amount of PCl.sub.5 in the presence of an inert organic solvent such as benzene and the subsequent conversion into 2-phenylethylene phosphonic acid by hydrolysis is known. The pure acid is obtained with a total yield of approximately 34% subsequently to recrystallization from dibromomethane.
The preparation of 2-phenylethylene phosphonic acid by chlorination of a mixture of styrene and phosphorus trichloride and subsequent hydrolysis is also known. However, it is unavoidable during this reaction that a large amount of the added styrene escapes conversion into phosphonic acid due to the formation of chlorination products.
Finally, a continuous process has been disclosed in German Published Application (DE-OS) No. 23 43 460 in which 2-phenylethylene phosphonic acid is prepared by the reaction of styrene with twice the molar amount of PCl.sub.5 and subsequent hydrolysis, POCl.sub.3 being used as solvent. This process has the disadvantage that considerable amounts of PCl.sub.5 as well as POCl.sub.3 must be used.